The 4CX250B is the classic RF Power tetrode valve for VHF and low UHF applications. The base is a special version of the B8F because it has to incorporate the forced air system, a matching ceramic chimney was made for the base, and the control grid was connected to the central metal spigot.
We have an example of the glass based 4X150A in the museum, this is almost identical but has a lower power rating.
The other side of the cooling fins carries more printed identification.
The anode to outer ring is filled with folded ribbon for a large surface area and this is clearly visible. All external metalwork is silver plated.
The B8F base. Pin 1 (g2) can be seen to be bonded to the lower metal shell. The locating key on the spigot is also clearly visible.
The thin metal tube envelope is 35 mm in diameter and, excluding the B8F base pins, is 46 mm tall.
Reference: Data-sheet. Type 4CX250B was first introduced in 1960.